Shengyang Zhou assured the American buyers that his Chinese-made weight-loss capsules were safe. Zhou was sure because he had a test — one person eats a whole bottle.

Such a person would have "maybe some stomach problem, but not die," he told undercover federal agents posing as buyers, according to court records.

Making sure customers didn't die was a big priority, Zhou told the agents at a meeting in Bangkok. "If the people die, I will be killed," he said.

Zhou had reason to soothe the buyers' concerns. His supplements were not simply made of herbs and other natural ingredients; they contained a pharmaceutical drug, an appetite suppressant called sibutramine that is no longer sold in the U.S. because of safety concerns.

The U.S. government's pursuit of Zhou, who is now serving time in federal prison, sheds light on an illegal practice that regulators say is increasingly common: adding drugs to products aimed at helping people lose weight, have better sex or bulk up, and labeling them as dietary supplements.

Such "supplements" have sickened customers young and old who do not realize they are consuming powerful medications. Some of the drugs are experimental and not approved for use under any circumstances.

"We consider it a very, very big problem," said Dan Fabricant, director of theU.S. Food and Drug Administration'sDivision of Dietary Supplement Programs. "If we went out there today, to the Web or to certain retail outlets, we would have absolutely no problem finding products that are tainted."

Operations like Zhou's operate largely off the FDA's radar, producing their products in unregulated factories overseas, using anonymous email addresses and aliases to sell them as dietary supplements over the Internet to U.S. middlemen, and shipping them through the mail in packages marked with innocuous labels like "rice."

Since 2008, the FDA has announced warnings or recalls on about 400 dietary supplements containing hidden pharmaceuticals, Fabricant said, adding that the recalls represent a fraction of what is out there. He called the real numbers "staggering."

Trade organizations for the dietary supplement industry say the problem is troubling for consumers and for responsible companies that make and sell supplements.

"Anytime criminals hijack the reputation of a legitimate industry, it is of great concern," Steve Mister, president and CEO of the Council for Responsible Nutrition, wrote in an email. "Consumers need to know that this kind of illegal activity is not indicative of the supplement industry."

Michael McGuffin, president of the American Herbal Products Association, wrote: "This is a limited problem in the context of the world of dietary supplements, since the tens of thousands of lawful supplement products do not contain these ingredients."

Mister called on the FDA to move aggressively against companies selling tainted products labeled as supplements. "Criminal acts deserve criminal punishments," he wrote.

Arrested in Hawaii in March 2010, Zhou is serving time in a federal prison in Youngstown, Ohio, after pleading guilty to trafficking and attempted trafficking in counterfeit goods and aiding and abetting that crime.

Contacted through his attorney, Zhou declined to be interviewed for this story. But documents from his case detail the inner workings of the underworld of a thriving illicit industry.

Zhou's journey from Audi Q7-driving millionaire to Inmate No. 11210-022 began in March 2009, when U.S. Customs and Border Protection agents intercepted a large brown package from China on its way to Colorado, according to court records. Its invoice identified its contents as "dry fruit."

Instead of fruit, the package contained 6,120 capsules of Super Slim and Meizitang, which were sold as dietary supplements but laced with sibutramine and phenolphthalein, a laxative suspected to be carcinogenic, according to court records.

The package led the agents to a Coloradan who had been selling the illicit diet pills on the Internet and gave up the names of his suppliers. Zhou, known to the Colorado distributor as "Tom," was one of them, according to a lengthy affidavit filed in court by special agent Russell Hermann, who was working with FDA's Office of Criminal Investigations.

Posing as buyers, Hermann and his colleagues met Zhou in Bangkok in February 2010, where they negotiated the sale of 200 boxes of the sibutramine-laced supplements, 2 Day Diet and Super Slim, and 1,000 boxes of counterfeit Alli, the popular weight-loss drug. The fake Alli also would contain sibutramine, he promised. The total price was set at $11,000.